The present invention relates to a method and apparatus for controlling a shot peening device including the measuring of the peening intensity during the operational cycle of the shot peening device.
Shot peening is a well known technique to impart surface compression stresses to a workpiece, such surface compression stresses serving to improve the useful life of the workpiece by increasing both fatigue and corrosion resistance under stress. Shot peening techniques involve the projection of shot elements, usually by compressed air, onto a specific zone of the workpiece surface. Shot elements are typically spherical balls and may be made of a variety of materials, such as steel, glass, or ceramics depending upon the particular material of the workpiece.
To produce workpieces of high quality with good reproducibility requires a reliable measurement means for determining the peening intensity. This is particularly true in workpieces which have aeronautical applications.
A known method for measuring shot peening intensity consists of making ALMEN type test pieces which are flat and are subjected to a shot peening jet. The shot peening intensity can be determined by measuring the stress-induced flexture of the test piece and consulting the known ALMEN measuring scale. This method entails much handling and requires test piece supports for each specific workpiece geometry, which are frequently complex, particularly workpieces relating to aeronautical engines. The measurement of the shot peening intensity can only take place before or after the workpiece has been treated. If the shot peening device undergoes any changes during the shot peening operation, unacceptable workpieces can result, since it is not possible to measure the shot-peening intensity during the operation of the shot peening device. For these reasons, this method has not been universally accepted.
Various attempts have also been made to monitor the shot peening operation by methods calling for additional quality checks, or involving controlling the parameters which determine the shot peening intensity. French Patent 2,312,775 describes a method in which the peened surfaces are inspected in relation to a control sample using a coating which radiates in a specific manner when subjected to fluorescent light. French Patent 2,627,414 describes a shot peening apparatus which includes a regulator for the supply pressure of the propellant gas and an independent control for the flow of shot elements.
U.S. Pat. No. 4,873,855 to Thompson discloses a system for using a magnetic densitometer to determine the mass of shot which, when combined with the mass flow rate, generates a signal representative of the average shot velocity.
U.S. Pat. No. 4,848,123, also to Thompson, discloses a system utilizing a force sensor in connection with the shot peening nozzle to sense the reaction force exerted on the nozzle. A signal representative of this reaction force is used to calculate the average shot particle velocity and mass flow rate.
French Patent 2,590,826 illustrates automation principles relating to shot peening equipment which includes an operational linkage between the means measuring the degree of covering of a test piece and means checking the characteristics of the shot peening jet.